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Constraints

NOT NULL CONSTRAINT -    Ensures that a column cannot have a null value. DEFAULT CONSTRAINT -    Provides a default value for a column when none is specified  UNIQUE CONSTRAINT -   Ensures that all values in columns are different  CHECK CONSTRAINT -   Makes sure that all values in a column satisfy certain criteria  PRIMARY KEY CONSTRAINT -   Used to uniquely identify a row in the table  FOREIGN KEY CONSTRAINT -   Used to ensure referential integrity of the data  Primary Key - is used uniquely to identify each row in a table . It can consist of one or more columns on a table . When Multiple columns are used on a table it is called composite key.  Foreign Key - Foreign key is a column or columns that references a column most often primary key of another table . The purpose of foreign key is to maintain referential integrity of the data. Pg admin  Data base - training - right click on training - query click  Always add semi colen to run the query  Int - integer  varchar - variable charact
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Trade and mercantile system

 Trade means exchange of goods and services 

Internal trade is trade within the country 

International Trade is trade outside the country. 

You need to adjust the political background when doing international trade.

Mercantile system - Was first coined by Adam smith that describes political economy in which nations encourages exports and restrain imports in order to increase financial wealth . 

It lies in achieving balance of Trade that would bring in more gold and silver into the economy because of possession of wealth. 

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